1. Field of the Invention
This invention relates to a method and an apparatus for producing a thermoplastic resin sheet, in which the obtained polypropylene resin sheet or film can be used for the packaging of food, medicine and so on.
2. Description of the Related Art
Of late years, as a transparentizing art of sheet (or film) made out of polypropylene resin, various processes with the use of a metallic endless belt have been proposed.
The greatest advantage of the use of the metallic endless belt is the point that high-glossing of both faces of the sheet can be effectively obtained to continuously transfer a mirror face of the metallic endless belt onto the surface of the sheet.
In order to increase the transparency of the polypropylene resin sheet, however, in addition to the aforementioned high-glossing, it is required that haze (the degree of cloud) inside the sheet is reduced to a low level.
Conventionally, various methods and apparatus for improving the transparency have been proposed.
For example, according to Japanese Patent Application Laid-open No. 6-170919, an apparatus for cooling and solidifying a fused sheet is disclosed, in which the metallic endless belt is provided to be wound on and between a metal roller and an elastic roller, covered with an elastic body on its surface, and after the sheet is guided into the area between the elastic roller and the cooling roller through the metallic endless belt, the sheet is, continuously, cooled while being pressed to a part of the circumference of the cooling roller.
In this case of the apparatus, the effect of immediately cooling the sheet pressed by the cooling roller and the metallic endless belt is insufficient, and in addition, hardness of the elastic body of the elastic roller is higher, so that the sheet is not touch-rolled on its face, with the result that a resin bank is produced or a fine transparency of the sheet cannot be obtained.
In Japanese Patent Application Laid-open No. 6-166089 and Japanese Patent Application Laid-open No. 6-170919, a method for cooling the sheet using the same method after the metallic endless belt is provided to be wound on and between the metal roller and the elastic roller covered with the elastic body on its surface is disclosed, however, the cooling temperature is higher and hardness of the elastic body is higher, so that the immediate cooling effect is insufficient, with the result that the transparent polypropylene sheet cannot be obtained without the use of raw materials consisting of a nucleating agent.
Further, according to Japanese Patent Application Laid-open No. 6-55613, a producing method of a thermoplastic resin sheet is disclosed, in which the sheet of the fusing state, consisting of a nucleating agent or the like, is guided into an area between the cooling roller and the metallic endless belt, and after the sheet is moved while being touched to the cooling roller and the metallic endless belt, the sheet is pressed against the cooling roller from the inside of the metallic endless belt by the aforementioned cooling roller.
In this case of the producing method, the immediate cooling of the sheet starts after the sheet is sandwiched between the cooling roller and the metallic endless belt, so that at the starting point of the immediate cooling, the face pressure is insufficient, with the result that it is difficult to obtain a fine effect of the immediate cooling and a fine surface gloss on the sheet. As a result, the transparency has tried to be improved by adding a nucleating agent or the like, but the degree of transparency may be insufficient if the nucleating agent or the like is not added.
A first aspect of the present invention is a producing method of a thermoplastic resin sheet, using a producing apparatus in which a metallic endless belt having a mirror face on the surface is provided to be wound on and between a first cooling roller, covered with an elastic member on the surface, and a second cooling roller, and a third cooling roller, having a mirror face on the surface and touching the first cooling roller through a molten thermoplastic resin sheet and the metallic endless belt, is provided to hold the molten thermoplastic resin sheet pressed with the metallic endless belt, which is characterized by guiding the molten thermoplastic resin sheet into the area between the first and third cooling rollers to touch the metallic endless belt, touching the first cooling roller, and the third cooling roller at approximately the same time, while the temperature of the metallic endless belt, directly touching the molten thermoplastic resin sheet, and the third cooling roller is retained to be less than 50xc2x0 C. and more than the dew-point; touch-rolling the face and cooling the molten thermoplastic resin sheet along a nip width (a length in MD (machine direction) of an elastically deformed area of the elastic member) of more than or the same as 5 mm between the first and third cooling rollers while the elastic member is elastically deformed by a pressing force between the first and third cooling rollers; and cooling, subsequently, the molten thermoplastic resin sheet by touch-rolling the molten thermoplastic resin sheet on the face against the third cooling roller by the metallic endless belt to cool.
The aforementioned molten thermoplastic resin sheet is a thermoplastic resin sheet just after being formed by being extruded from an extruder.
The molten thermoplastic resin sheet in the present invention should be recognized as to also mean a film-state molten thermoplastic resin, too.
As the preferable thermoplastic resin, polyethylene, polypropylene, polyvinyl, chloride, polyvinyl, alcohol, polyethylene terephthalate, polyarylate, polyethylene-2,6 naphthalate, polyethylene-xcex1, xcex2, bis (2-chlorophenoxyethane-4,4xe2x80x2-dicarboxylate), polyphenylene oxide, polyester, polyphenylene sulfide, polyether ether ketone, aromaticpolyamide, polyarylate, polyimide, polyamide imide, polyether imide, syndiotactic polystyrene, biphenylether ketone, or the like can be used.
The recommended thermoplastic resin is a polypropylene resin.
As the aforementioned polypropylene resin, a homopolymer of polypropylene, a copolymer (random, block) with ethylene, a composition thereof or the like can be used. A random copolymer with ethylene is especially outstanding in the transparency. Incidentally, in the resin, a nucleating agent, it may consist of a sorbitol type, benzoic aluminum type, sodium type or the like.
And the sheet may be a single layer of thermoplastic resin or multi-layers including a layer of thermoplastic resin.
In the present invention, when the temperature of the metallic endless belt and the roller in order to cool the molten thermoplastic resin sheet exceeds 50xc2x0 C., a fine transparency cannot be obtained. More preferably, it is less than 30xc2x0 C. Further, when it is lower than the dew-point, a water-droplet spot is produced on the sheet.
By guiding the molten thermoplastic resin sheet into the area between the first and third cooling rollers to touch the metallic endless belt, touching the first cooling roller, and the third cooling roller at approximately the same time, the molten thermoplastic resin sheet can be touch-rolled and cooled at the same time, resulting in the improved transparency of the thermoplastic resin sheet. It can be assumed, when the molten thermoplastic resin sheet is touched earlier to either the metallic endless belt or the cooling roller, before the mirror face is transferred onto both faces of the sheet, the cooling and caking of the molten thermoplastic resin sheet is advanced.
In the present invention, it is advisable that the degree of the surface roughness of the mirror face of the metallic endless belt and the roller is less than 0.5 S, more preferably less than 0.4 S (Rmax).
As for the material of the endless belt, stainless, carbon steel, titanium alloy or the like can be used. Preferably, martensitic precipitation hardening stainless steel (such as SUS 630 or 631 type) or maraging steel, with fatigue limit against alternating stress (average stress=0) is more than xc2x1600 N/mm2, preferably more than 650 N/mm2. The thickness of the endless belt is selective, more preferably more than 0.3 mm from view of strength, and not exceeding 1 mm considering flexibility. Preferably, the endless belt is a seamless belt. Furthermore, it is preferable for the thickness (t) of the belt and the diameter (d) of the roller to be defined as 500xe2x89xa6d/txe2x89xa6900.
Being thus structured, the size of the belt mechanism body can be reduced, thereby reducing the cost and space for the belt mechanism and facilitating handling thereof (in an ordinary belt mechanism a belt of more than 1 mm and a roller having diameter of 1000xe2x89xa6d/t (d: diameter of the roller, t: thickness of the belt) are employed).
As for the material of the aforementioned elastic member, fluorine-type rubber, silicone-type rubber, EPDM or the like can be used. It is advisable that the thickness of the elastic member is more than 3 mm in order to obtain fine face pressure with the elastic deformation.
Preferably, the sheet is separated after being moved out from the nip between the third cooling roller and the belt with the sheet retained on the belt. The sheet is preferably cooled on the belt until the surface temperature thereof reaches 50xc2x0 C. when being separated.
In accordance with such process, the separation is conducted after the sheet being sufficiently cooled, thereby preventing a damage on the sheet.
On the other hand, it is also preferable to separate the sheet after being moved out from the nip. Being thus structured, damage on the surface of the belt can be prevented even when (1) a resin of which crystallization comes fast or (2) a resin being largely contracted by crystallization, is employed.
In the present invention, the aforementioned molten thermoplastic resin sheet is touch-rolled on its face and cooled with the elastic deformation of the aforementioned elastic member, resulting in the improved efficiency of the transfer of the mirror face and the cooling.
A second aspect of the present invention is a producing method of the thermoplastic resin sheet, using a producing apparatus in which a first metallic endless belt, having a mirror face, wound on and between a first cooling roller, covered with an elastic member on the surface, and a second cooling roller and a second metallic endless belt, having a mirror face, wound on and between a third cooling roller and a fourth cooling roller are provided to run along side each other to sandwich a molten thermoplastic resin sheet, and an added-pressure means for both metallic endless belts is provided in a section where both metallic endless belts run along side each other to sandwich the molten thermoplastic resin sheet, which is characterized by guiding the molten thermoplastic resin sheet into the area between the first and second metallic endless belts to touch the first endless belt, touching the first cooling roller, and the second metallic endless belt, touching the third cooling roller, at approximately the same time while the temperature of the metallic endless belt directly touching the molten thermoplastic resin sheet is retained to be less than 50xc2x0 C. and more than the dew-point; touch-rolling the face and cooling the molten thermoplastic resin sheet by the first and third cooling rollers while the elastic member is elastically deformed along a nip width (a length in MD (machine direction) of an elastically deformed area of the elastic member) of more than or the same as 5 mm by a pressing force between the first and third cooling rollers; and cooling, subsequently, the molten thermoplastic resin sheet by touch-rolling the formed molten thermoplastic resin sheet on the face by the added-pressure means in a section where both metallic endless belts run along side each other.
In the second aspect of the present invention, the third cooling roller may be covered with the elastic member on the surface of the third cooling roller.
That is to say, the first and third cooling rollers of the invention are covered with the elastic member on both surfaces.
In the second aspect of the present invention, the added-pressure means for both metallic endless belts can be at least a pair of rollers located to sandwich both metallic endless belts.
The number of pairs of rollers can be decided based on the length between the first and second rollers and between the third and fourth rollers.
The aforementioned pair of rollers may be located at a position to be opposed to each other by sandwiching both metallic endless belts or may be located at a position to be away from each other not to oppose to each other by sandwiching both metallic endless belts.
In the second aspect of the present invention, the added-pressure means for both metallic endless belts may be a roller located to cause both metallic endless belts to curve between the first and second rollers or between the third and fourth rollers.
Both metallic endless belts are selectively located to curve between the first and second rollers or between the third and fourth rollers, and in either case, the same effect of the added-pressure can be obtained.
In the first aspect of the present invention or the second aspect of the present invention, it is advisable that when the molten thermoplastic resin sheet is touch-rolled on the face with the elastic deformation of the elastic member, the face pressure is defined as 0.1 MPa-20.0 MPa; and when the molten thermoplastic resin sheet is touch-rolled on the face without the elastic deformation of the elastic member, the face pressure is defined as 0.01 MPa-0.5 MPa.
When the face pressure is lower than 0.1 MPa or 0.01 MPa described above, the efficiency of transfer of the mirror face and the cooling become reduced. When the face pressure is higher than 20.0 MPa or 0.5 MPa described above, tension of the belt is higher, so that it is not preferable from the view of the life of the belt.
Preferably the hardness of the elastic member is less than 60 degrees (in accordance with JIS K6301 A) and the thickness thereof is less than 5 mm. Being thus structured, elastic deformation along a nip width of more than or the same as 5 mm can be easily and securely conducted.
A producing apparatus of a thermoplastic resin sheet, relating to a third aspect of the present invention, is characterized by providing a metallic endless belt having a mirror face, which is wound on and between a first cooling roller, covered with an elastic member on the surface of the metallic endless belt, and a second cooling roller; and a third cooling roller having a mirror face and touching the first cooling roller through a molten thermoplastic resin sheet and the metallic endless belt, which is provided to hold the molten thermoplastic resin sheet pressed by the metallic endless belt.
That is to say, the invention is the producing apparatus used in order to carry out the producing method of the first aspect of the present invention.
A producing apparatus of a thermoplastic resin sheet, relating to a fourth aspect of the present invention, is characterized by providing a first metallic endless belt having a mirror face, which is wound on and between a first cooling roller covered with an elastic member on the surface of the first metallic endless belt; a second metallic endless belt having a mirror face, which is wound on and between a third cooling roller and a fourth cooling roller, in which the first metallic endless belt and the second metallic endless belt are provided to run along side each other to sandwich a molten thermoplastic resin sheet; and an added-pressure means for adding pressure to both metallic endless belts in a section where both metallic endless belts run along side each other to sandwich the molten thermoplastic resin sheet.
That is to say, the invention is the producing apparatus in order to carry out the producing method of the second aspect of the present invention.
In the fourth aspect of the present invention, the third cooling roller can be covered with the elastic member on the surface of the third cooling roller.
In the fourth aspect of the present invention, the added-pressure means for adding pressure to both metallic endless belts can be at least a pair of rollers located to sandwich both metallic endless belts.
In the fourth aspect of the present invention, the added-pressure means for adding pressure to both metallic endless belts can be a roller located to cause both metallic endless belts to curve between the first and second rollers or between the third and fourth rollers.
In the third aspect of the present invention or the fourth aspect of the present invention, it is advisable that the elastic member is less than 95 degrees, preferably 60 degrees of the hardness (reference to a type of JIS K6301 A).
When the hardness is more than 95 degrees, the elastic force is smaller, so that a resin bank is easily produced when the molten thermoplastic resin sheet is touched either by both the cooling roller and the metallic endless belt or by both of the metallic endless belts at approximately the same time.